Effective Field Equations of the Quantum Gravitational Back-Reaction on Inflation

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Quantum gravitational back-reaction offers the potential of simultaneously resolving the problem of the cosmological constant and providing a natural model of inflation in which scalars play no special role. In this model inflation begins because the cosmological constant is not unnaturally small. It ends through the accumulated gravitational interaction between virtual gravitons which are ripped apart by the inflationary expansion. Although perturbative techniques can be used to study the effect as long as it remains weak, they break down when back-reaction begins to exert an appreciable effect on the expansion rate. In this talk I argue that the end of inflation is sudden and that there is actually an overshoot into deflation. (This incidentally provides a very efficient mechanism for reheating.) The subsequent evolution can be understood in terms of a competition between the opening of the past light cone and the formation of a thermal barrier to the persistence of correlations from during the period of inflation.